1. Field of the Invention
The present invention relates generally to a lock-up clutch control system for an automatic power transmission for an automotive vehicle. More specifically, the present invention relates to a lock-up clutch control system for a fluidly operated power transmission unit used in an automatic automotive power transmission, wherein a required response of vehicular acceleration can be provided even when a slip lock-up control of the fluidly operated power transmission unit is performed.
2. Description of the Background Art
One of previously proposed lock-up clutch control systems is exemplified for example, by Japanese First Patent Publication No. 61-206868 published on Sept. 13, 1986.
In the system of this publication, a torque converter is selectively operated under a torque converter mode, a slip lock-up mode and a full lock-up mode according to a predetermined control schedule which is determined in terms of an engine load and a vehicle speed. In the torque converter mode, a turbine runner is hydraulically operated by the movement of a pump impeller, while, in the full lock-up mode, the pump impeller is directly and mechanically connected to the turbine runner through a lock-up clutch. The slip lock-up mode is a half-clutch mode or a transitional mode for smoothly shifting from the torque converter mode to the full lock-up mode.
In the above-noted background system, however, even if an accelerator pedal is depressed to accelerate the vehicle during the slip lock-up mode, the slip lock-up mode continues according to the above-noted control schedule. Since the slip lock-up mode is the half-clutch mode where the slip is permitted with the lock-up clutch being half engaged, this becomes a load to the engine to prevent an engine speed from increasing so that an acceleration response corresponding to the operation of the accelerator pedal is not attained.